An equalizing unit of a drive train of a motor vehicle with a housing and with drive members comprises an input shaft and at least one first output shaft. The first output shaft can be coupled to a second output shaft or to the input shaft or to an intermediate shaft driven by the input shaft via a clutch device, which is to be supplied with oil via a delivery device directly or indirectly driven via the drive members.
The construction of an equalizing unit described above is known from German publication DE 10 2008 002 844 A1. There, a drive train for a motor vehicle is shown, including a transverse equalizing unit without differential wherein the output shafts for driving the drive wheels are driven via two individually activatable side shaft clutches. The side shaft clutches are formed by disc packages which for cooling and lubrication have to be oiled. The outer discs of the disc package are connected in a rotationally fixed manner to an intermediate shaft driven by the input shaft—in DE 10 2008 002 844 A1 this is the crown wheel carrier shaft—and through their special configuration deliver the oil to the outside into an oil collection pocket provided in the housing, from where the oil via housing bores flows back to the interior of the disc package where it is again collected by the discs so that—based on a disc package of a side shaft clutch—an internal oil circuit is formed.
Because of this internal oil circuit a large part of the oil volume expended for the cooling and lubrication remains in the described internal oil circuit for a long time without the oil volume being sufficiently mixed with “fresh” oil or replaced with such. Oil volume, which has only just absorbed heat between the clutch discs, is directly returned to the discs again. The clutch temperature therefore rises disproportionally to the cooling output that could be achieved with the oil quantity present in the equalizing unit. The disproportionate heating of the clutch brings disadvantages with regard to wear characteristics and thus the lifespan, and also is problematic with regard to response and control behavior.
A further disadvantage of the equalizing unit shown in DE 10 2008 002 844 A1 is that the clutch packages of the side shaft clutches stand in the oil in order to collect it and deliver it to the oil collection pocket. The concomitant splashing losses increase the dissipation of the drive train.
In addition to undesirable dissipation, even in the case described in DE 10 2008 002 844 A1 where the secondary part of the drive train is decoupled from the drive wheels and the side shaft clutches are fully opened, the secondary drive wheels roll on the street in driving mode and drag the inner disc carrier and the inner discs connected with the latter in a rotationally fixed manner. The inner discs, however, are in permanent contact with the oil since they are immersed in it. Not only the region of the extremely narrow air gap (gap width approximately 0.1 mm) between inner and outer discs which is directly located in the oil is filled with oil, but the oil is additionally delivered through the rotating inner discs into the intermediate space between inner and outer discs of the disc clutch which do not stand in the oil. The fluid friction or hydrodynamic friction created as a consequence in turn transmits drag torque, acting from the secondary drive wheels, to the inner discs, and then to the outer discs, and because of this the friction drives the outer discs. However, as soon as these start to rotate these in turn deliver oil into the previously described internal oil circuit and on their part thus again increase the delivered oil volume and thus the friction loss or dissipation. The undesirable effect amplifies itself.
Tests have shown that because of this effect the secondary drive train, contrary to the assumptions made in DE 10 2008 002 844 A1, does not come to a halt even when it is decoupled from the primary part of the drive train. Through the low-friction configuration of the surfaces of the inner discs proposed in DE 10 2008 002 844 A1, the negative consequences of this effect can only be reduced to a very minor degree. In addition, the low-friction surfaces of the inner discs result in a significantly poorer response and control behavior of the clutch package.